function SIR=fSIR(t,X)
    global BETA;
    global GAMMA;
    
    S = X(1);
    I = X(2);
    % R = X(3); unused
    
	SIR(1) = -BETA * S * I;
    SIR(2) = BETA * S * I - GAMMA * I;
    SIR(3) = GAMMA * I;
end